1. Field of the Invention
The purpose of this invention is to provide a process for increasing the frequency of operation of a magnetic circuit and a corresponding magnetic circuit.
It has applications in the manufacture of magnetic components, especially inductive components (typically inductors, either single or multiple, or being part of a network of elementary components integrated into the same chip), in the manufacture of transformers, magnetic-field sensors, or instruments for measuring a quantity related to a magnetic field, magnetic recording heads, etc . . .
2. Discussion of the Background
In inductive components (inductors, transformers, magnetic heads, etc . . . ), it is advantageous to channel the magnetic flux by means of a high-permeability magnetic circuit as this permits either a gain in performance for a given size or a reduction in size for a given performance.
In macroscopic radio-frequency components, magnetic circuits are generally made of solid ferrite while, in integrated components, stacks of thin layers of ferromagnetic alloy (typically Fe—Ni) and insulating material are more frequently used. The development of such integrated components is presently underway through active research in many laboratories.
The miniaturization of these components makes it possible to increase their working frequency by reducing, in particular, propagation and induced-current phenomena.
The performance of insulator/alloy composites in the form of thin layers is much better than that of ferrite components and makes it possible to consider operation at frequencies extending well beyond the radio-frequency range. Nonetheless, these materials have their own limitations, related either to fundamental phenomena or to the technology used. Two limiting phenomena related to technology are skin effect and dimensional resonance. Both have the effect of reducing the effective permeability of the composite and altering its frequency response.
The first one can be avoided (or limited) by, as is done conventionally, choosing a thickness for the magnetic layers in the stack much smaller than, or on the same order of size as, the skin depth. As an example, the skin thickness is 0.2 μm at 1 GHz for the Fe—Ni alloy.
The second one, related to dimensional resonance, is associated with the electromagnetic propagation inside the composite in directions parallel to the layers. It can be limited, in one case, by maintaining a sufficient thickness of insulating material between the magnetic layers (to the detriment of the packing factor) and, in the other case, by limiting the side dimensions of the magnetic circuits or the cores.
Consequently, for a frequency of 1 GHz, the width of the Fe—Ni magnetic circuit or magnetic core should be much less than 700 μm, a condition just about compatible with integration concerns.
Another limitation, unrelated to the technology involved and more fundamental in nature, corresponds to the phenomenon of gyromagnetic resonance. The frequency of this resonance constitutes, as is known, an upper limit in the usable frequency range, knowing that at frequencies below this resonance the relative permeability is practically constant and equal to its static value. It is well known that, in an alloy with a given composition, it is possible, by means of simple heat treatments, to vary the permeability and the resonant frequency. Consequently, the limitation due to gyromagnetic resonance is not expressed only in terms of frequency. It can be shown that the product μ2·fr2, where μ2 is the static value of the permeability and fr the gyromagnetic resonant frequency, is constant for an alloy with a given composition when, through treatment after deposit, μ2 and fr are modified at the same time. This product thus constitutes a merit factor for the material, which depends only on its composition. It can be shown that it depends practically only on the spontaneous magnetization of the alloy. For the Fe—Ni alloy:μ2·fr2=1300 GHz2For a composite whose packing factor is η, there is simply:μ2·fr2=η·1300 GHz2The existence of such a relationship shows that μ2 and fr cannot be modified independently.
In particular, operation at higher and higher frequencies requires a reduction in magnetic permeability.
For a given working frequency f, an attempt is thus made, in general, to condition the material in such a way that the resonant frequency fr lies well above f. This assumes that the material can be adapted to the application under consideration. The resonant frequency could be modified by a heat treatment after deposit. But this technique has drawbacks: compatibility with the device's manufacturing processes is not assured and, in any case, the variations obtained remain small.
The purpose of the invention is to overcome these drawbacks.